The production of glass-ceramic articles was first detailed in U.S. Pat. No. 2,920,971 (Stookey). That patent defines the preparation thereof as comprising three fundamental steps. First, a glass forming batch commonly containing a nucleating agent is melted. Second, that melt is cooled to a temperature below the transformation range thereof and, simultaneously with said cooling, a glass article of a desired configuration is shaped therefrom. (As employed herein, the transformation range is characterized as that temperature at which a molten mass becomes an amorphous solid, that temperature being deemed to reside in the vicinity of the annealing point of the glass. Third, that glass article is subjected to a heat treatment schedule to crystallize the glass in situ. Customarily, the heat treatment schedule consists of two steps: (1) the glass article is initially heated to a temperature somewhat above the transformation range for a sufficient length of time to generate nuclei therein; and thereafter (2) the temperature is raised to above the annealing point and frequently above the softening point of the glass for a sufficient length of time to cause the growth of crystals on the nuclei.
Glass-ceramic articles are typically highly crystalline, the crystals being fine-grained, substantially uniform in size, and homogeneously dispersed throughout the minor amount of residual glassy phase. Because the crystals are developed in situ in a precursor glass body, glass-ceramic articles are free of voids and non-porous.
Whereas U.S. Pat. No. 2,920,971 describes the formation of opaque glass-ceramic articles, further research led to the preparation of transparent articles. One transparent glass-ceramic being currently marketed commercially by Corning Incorporated, Corning, New York is cookware under the trademark VISIONS . That cookware lies within the scope of U.S. Pat. No. 4,018,612 (Chyung), which patent defines compositions within a narrow range of the Li.sub.2 O-MgO-ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 system nucleated with a combination of TiO.sub.2 and ZrO.sub.2 to produce highly crystalline articles containing .beta.-quartz solid solution as the predominant crystal phase.
Recent marketing studies have indicted a widespread customer interest in cookware fashioned from a transparent glass-ceramic body exhibiting a soft blue or light gray tint. U.S. Pat. No. 4,018,612 discusses the optional incorporation of glass colorants in the parent glass composition, explicitly referring to chromium, cobalt, copper, iron, manganese, nickel, and vanadium. U.S. Pat. No. 4,192,688 (Babcock et al.) also describes the inclusion of glass colorants in the precursor glass compositions for glass-ceramic articles. The latter patent, however, observes that the colors developed in a glass-ceramic body are frequently quite different from those conventionally generated in glass bodies. Moreover, the color of a transparent glass-ceramic using the same coloring agent will frequently be different from that observed in an opaque glass-ceramic article having the same base composition. As an illustration of those phenomena, cobalt has long been recognized as conferring a blue color to glasses, but in a transparent glass-ceramic article the color becomes lavender, and in an opaque glass-ceramic of the same composition the color displayed is a very pale blue. In another example, a combination of cobalt and nickel imparts an amber color to the parent glass article, but yields a cranberry red hue in the transparent glass-ceramic article produced therefrom and a blue shade in the opaque glass-ceramic article derived therefrom.
U.S. Pat. No. 4,084,974 (Beall et al.) discloses the production of colored transparent glass-ceramic articles containing .beta.-quartz solid solution as the predominant crystal phase from parent glasses having base compositions within the lithium aluminosilicate system nucleated with TiO.sub.2. In that patent, a carbonaceous reducing agent (expressly noting carbon, starch, and sucrose) is added to the glass forming batch so that, during melting, some of the Ti.sup.+4 ions will be reduced to Ti.sup.+3 ions. Thereafter, when the glass body is crystallized in situ, the resulting transparent glass-ceramic will display a blue-to-black color, depending upon the extent of titanium reduction.
Although the operability of that procedure could be shown repeatedly on a small laboratory scale, it proved difficult to control in large scale commercial production. That is, problems in fining of the glass melt and variations in physical properties such as the coefficient of thermal expansion were experienced.
Accordingly, the principal objective of the present invention was to prepare transparent glass-ceramic articles containing .beta.-quartz solid solution as the predominant crystal phase, exhibiting a linear coefficient of thermal expansion (25.degree.-600.degree. C.) less than 10.times.10.sup.-7 /.degree. C., preferably less than 6.times.10.sup.-7 /.degree. C., and demonstrating a light blue or gray color without the use of a carbonaceous reducing agent.